The objective of this proposal is to understand the molecular mechanisms that modulate toxin-induced hepatocyte injury and death. Recent investigations have demonstrated that this cell death is not merely the result of the biochemical effects of toxins, but rather is a process actively regulated by cellular signaling cascades and gene expression. Critical to the outcome from toxin-induced liver injury in vivo are death signaling pathways activated by tumor necrosis factor-alpha (TNF) and reactive oxygen species (ROS). By examining the molecular events following injury, we have demonstrated a central role for the transcription factors NF-?? and AP-l in the hepatocyte's death response to TNF and ROS. The central hypothesis proposed is that hepatocyte death from TNF and ROS results from the inactivation of NF-??- and mitogen-activated protein kinase (MAPK)-dependent signaling cascades that normally down-regulate c-Jun NH2-terminal kinase (JNK). This failure to inactivate JNK leads to a c-Jun-dependent AP-1 induction of a common cell death pathway. This application contains three specific aims that will determine how JNK/c-Jun becomes activated in hepatocytes in response to injurious stimuli, and the mechanism by which c-Jun functions to induce hepatocyte death. Specific Aim 1 will test the hypothesis that hepatocyte resistance to TNF-induced death is dependent on NF-??-mediated down-regulation of JNK. Specific Aim 2 will test the hypothesis that extracellular signal-regulated kinase (ERK) activation is a critical cytoprotective response that prevents cell death from ROS through phosphatase-mediated inhibition of JNK. Specific Aim 3 will test the hypothesis that hepatocyte death from TNF and ROS is triggered by sustained AP-1 activation that: (a) results from JNK-induced c-Jun overactivation and a lack of JunB/JunD expression; and (b) promotes cell death through the transcriptional induction of genes that regulate the mitochondrial death pathway.